Conventional DVD front end chips implement several detected signals that are used by control and/or servo circuitry. The detected signals include Blank, Defect and Ripple (or Mirror) signals. The Blank signal tells the control circuitry whether the read laser spot is currently on an un-recorded area of a disc. The Defect signal tells the control circuitry whether the laser spot is currently on a defective area of a disc. The Ripple (or Mirror) signal tells the servo circuitry whether the laser spot is currently on a land or mark (groove) region.
In one conventional approach for generating the Blank and Defect signals, analog top (or peak) and bottom holds are used on a DC-coupled RF signal. When a difference between an output of the top hold and an output of the bottom hold is below a preset threshold, a defect/blank condition is detected. By comparing the output of the bottom hold to another preset threshold, a blank condition is detected when the output of the bottom hold is greater than the preset threshold and a defect condition is detected when the output of the bottom hold is lower than the preset threshold. However, the above approach has a disadvantage because the peak and bottom hold time constants are relatively large at low speed and therefore, involve a large capacitor either on chip or external to the chip. Also, the preset threshold has to be set differently according to media type and speed. The non-uniformity of a disc can some times cause unreliable detection.
In another conventional approach for generating the Blank and Defect signals, an AC-coupled RF signal is sliced with a preset non-zero threshold. The sliced output is sent to several programmable one-shot circuits to filter out noise and the outputs of the one-shots are combined. The combinational output of the one-shots can be used to detect a non-blank condition. However, the combinational output of the one-shots cannot be used to detect the defect condition.
In one conventional approach to detect the defect condition, the DC-coupled RF signal is low pass filtered to generate an analog MBS signal and converted into a digital MBS signal. The digital MBS signal is then compared to a fixed threshold. In another conventional approach to detect the defect condition, a higher bandwidth low passed MBS signal and a lower bandwidth low passed MBS signal are generated and a comparison is performed, where the lower bandwidth low pass filter acts like an adaptive threshold. However, defect detection is relatively slow. Because the defect detection is slow, a servo and read channel clock can already have moved too much by the time the defect is detected.
In a conventional approach for generating the ripple signal, a bottom hold is applied to the DC-coupled RF signal and an output of the bottom hold is compared to a preset threshold. However, the bottom hold time constant is relatively large at low speed and, therefore, involves a large capacitor either on chip or external to the chip.